As to a projector, the research and development of a product using a solid light source in place of a discharge lamp have been actively conducted with the aim of improving performance and reducing size and cost. For example, a rear projection TV and a pocket projector employing an LED light source have been produced on a commercial basis.
As to the solid light source for the projector, a laser light source is regarded as being promising along with an LED. Everybody recognizes that the laser light source has a high potential capacity as a light source, but a projector employing the laser light source has not been put into practical use. This is not only because an inexpensive semiconductor laser emitting green light has not been put into practical use but also because various regulations are required in terms of the characteristics of a laser light source.
A beam scan type projector that scans a laser beam horizontally and vertically by using a MEMS scanner to display an image can be reduced in size to an extent that cannot be considered in comparison with an existing projector. However, the beam scan type projector needs to observe safety standards stipulated in the international laser safety standards IEC60825 and the like. In the classification by this IEC60825, the light output of the light source is regulated to a small level. For this reason, it is considered to be difficult to realize a projector having brightness high enough for practical use or the same brightness as a conventional projector using a discharge lamp. In this regard, an illumination intensity that is safe even if the laser beam is directly incident on a human eye is stipulated in each class, and the stipulation is different according to the conditions under which the laser beam is viewed.
On the other hand, a front projection type projector that does not directly scan a laser beam is known (for example, Japanese Unexamined Patent Publication No. 2008-58454). This is a projector of the type in which a laser light is applied to a two-dimensional micro display such as a liquid crystal light valve and a DMD (digital mirror device) and in which an image displayed on the micro display is magnified and projected by the use of an optical system such as a projection lens.
It is considered that a projector of this type can realize higher brightness than the beam scan type projector.
In this regard, when the front projection type projector employing a laser light source is used, the most dangerous state as regards safety is considered to be brought about when a human eye comes closest to a projection lens.
Usually, a micro display as small as an inch or less is used in many cases, so that it can be considered that the size of a light beam passing through a lens on a side nearest to the exit of the projection lens, that is, a side to which a human eye comes closest is larger than 7 mm which is an average diameter of the pupil of the human eye. For this reason, it is only necessary to discuss the safety of power of a laser beam that can enter the diameter of 7 mm. Hence, it is only necessary to make a calculated AEL (accessible emission limit) consistent with a safety class that a projector is desired to satisfy.
In the projector using the laser light source, especially in the case of a front projection type projector, a zoom lens is mounted as a projection lens in many cases from the viewpoint of convenience to users and superiority of a product. For example, in a case where the projector installation distance is not long enough to match the size of a screen of a projector, if the zoom magnification of the zoom lens is changed, an image projected to the screen can be adjusted to the size of the screen. In this description, when an image of a specified size is projected to the screen, the setting state of the focal distance f of the zoom lens in which a projection distance to the screen can be made shortest is referred to as a “wide state”, whereas when an image of the same size as in the wide state is projected to the screen, the setting state of the focal distance f of the zoom lens in which a projection distance to the screen can be made longest is referred to as a “telescopic state”.
Usually, in the zoom lens of the projector, the position and the angle of a light beam passing through a light emitting surface of the zoom lens are changed in the process in which the wide state is changed to the telescopic state. Hence, an area which is a portion of the light emitting surface of the zoom lens and which is seen as bright (an area of a portion through which a laser beam passes) is changed along with an adjustment of magnification of the zoom lens. The size of this area is larger in the wide state and is smaller in the telescopic state (see FIG. 1).
As a result, under conditions in which the laser output of the light source is constant, the difference in safety, specifically, the difference in AEL is caused between being in the wide state and in the telescopic state.
It is in the telescopic state in which a laser power density is higher than in the wide state that the laser beam is dangerous for a human eye. For this reason, if the laser output is designed to be the AEL that satisfies a safety class or less in the telescopic state, the safety class can be guaranteed over the full range in which the magnification of the zoom lens is changed. However, this measure cannot make the best use of potential capacity in the wide state in which there is an allowance for the AEL. In other words, although brightness in the wide state can be further increased legally up, the brightness cannot be increased.